1. Field of Invention
The invention relates to polymer particles, particularly latex polymer particles, that may be used to form toner. The resulting toners can be selected for known electrophotographic imaging and printing processes, including digital color processes, and are especially useful for imaging processes, specifically xerographic processes, which usually require high toner transfer efficiency, such as those having a compact machine design without a cleaner or those that are designed to provide high quality colored images with excellent image resolution and signal-to-noise ratio, and image uniformity, and for imaging systems wherein excellent glossy images are generated.
2. Description of Related Art
Numerous processes are known for the preparation of toners. For example, in conventional processes, a resin is melt kneaded or extruded with a colorant, particularly a pigment, and the product thereof is micronized and pulverized to provide toner particles. The toner particles formed by this process generally have an average volume particle diameter of from about 7 microns to about 20 microns and a broad geometric size distribution of from about 1.4 to about 1.7. As a result, it is usually necessary to subject the aforementioned toner particles to a classification procedure such that a geometric size distribution of from about 1.2 to about 1.4 is attained.
There are also several so-called chemical processes for making toner, among them is the aggregation/coalescence process for making toner particles. In this process, narrow particle size distribution can be achieved without classification. In this process, the resin is prepared as a water based dispersion of sub-micron sized polymeric particles (polymeric latex), which are then aggregated with pigment particles of sub-micron size to the desired toner size and are then coalesced to produce pigmented toner particles.
U.S. Pat. No. 5,853,943, which is herein incorporated in its entirety by reference, is directed to a process for preparing a latex polymer by emulsion polymerization. In this process, the latex polymer is formed by first forming a seed polymer. To form toner from the latex polymer, U.S. Pat. No. 5,853,943 discloses blending the latex with a colorant dispersion; heating the resulting mixture at a temperature below or equal to the Tg of the polymer in the latex to form toner sized aggregates; and heating the aggregates at a temperature at or above the Tg of the polymer to coalesce or fuse the components of the aggregates.
A wide variety of polymer types are used in forming the polymer particles of toner. The polymers include both homopolymeric and copolymeric compositions, such as styrene-butadiene-acrylic acid copolymers, styrene-butyl acrylate-acrylic acid copolymers and acrylic homopolymers.
By selecting various homopolymers and copolymers, toners can be generated that possess specific chemical, mechanical and/or triboelectrical properties. In particular, toners with a low minimum fixing temperature (MFT) are desired to, for example, reduce the energy requirements of the printers and copiers, and to further extend the lifetime of the fuser rolls. However, reducing the MFT of the toner may cause other properties of the toner to be diminished.
The use of styrene-butyl acrylate latexes in emulsion aggregation toners is known. Toners formed with these latexes provide excellent image gloss and reasonably good fusing temperature. However, the transfer of toner from the xerographic images to a plasticized poly(vinyl chloride) (PVC) surface, such as a notebook cover, may occur. Commonly referred to as vinyl offset, this problem is related to the diffusion of plasticizer from the vinyl into the polymer. In particular, shift of a plasticizer such as dioctyl phthalate (DOP) or dibutyl pthalate (DBP) contained in soft PVC to the toner causes the viscosity of the toner to decrease so that the toner adheres to the PVC. The type of plasticizer used in the PVC affects vinyl offset. However, the polymer structure of the toner resin and its molecular weight appears to also be a major factor affecting vinyl offset.
The present invention is directed to the use of a copolymer of styrene and an acrylic ester as toner resin, particularly as formed by emulsion aggregation. In the present invention, the acrylic ester has an alkyl group with three or fewer carbon atoms or the acrylic ester is an aromatic acrylic ester.
Toner containing the above-mentioned copolymer is particularly useful for low melt/high gloss applications and has superior resistance to vinyl offset as compared to copolymers in which the acrylic ester is a butyl acrylate. However, basic toner characteristics such as tribocharge, image fixing performance and gloss are substantially maintained and/or improved.